1. Field of the Invention
The present invention, in certain aspects, is directed to seismic survey systems and methods in which two or more seismic sources are fired simultaneously, or significantly close together temporally, but which is, in one aspect, significantly spatially separated, and resulting seismic data is processed meaningfully utilizing data generated by both (or more) seismic sources.
3-D marine seismic surveys entail towing a swath of elongated seismic sensor arrays. Subsea formations are acoustically illuminated to produce seismic reflection data that are detected and processed by the arrays and associated ancillary equipment. In the presence of steeply-dipping subsea formation, this invention corrects the non-uniform illumination of the formations due to the backward geometry caused by the steeply-dipping wavefield trajectories.
2. Description of Related Art
The prior art discloses seismic survey systems and methods employing two or more seismic sources firing simultaneously. In order to make meaningful use of resultant seismic data, each source is initially encoded differently e.g. signals at different frequency bands or phases (orthogonal)! so that resulting seismic data contains a signature indicating to which source the data is related. Such encoding requires corresponding decoding when processing the data. Often, in actual practice, the level of separation achievable is not satisfactory. Also, encoding is impractical for some source configurations.
There has long been a need, now recognized and addressed by the present invention, for seismic survey methods in which multiple seismic sources firing simultaneously or temporally close together may be used effectively and efficiently. There has long been a need for such methods which do not require individual encoding or other separate identification of each of two or more seismic sources.
In 3-D marine operations, a seismic ship tows a swath including a plurality of parallel seismic streamer cables along a desired line of survey, the cables being submerged by a few meters beneath the water surface. The number of cables that make up a swath depends only on the mechanical and operational capabilities of the towing ship. There may be six or more such cables, spaced about 50 to 100 meters apart. The respective cables may be up to 3000 meters long.
Each streamer cable typically includes about 120 spaced-apart seismic detector groups. Each group consists of one or more individual interconnected detectors, each of which services a single data channel. The group spacing is on the order of 25 to 50 meters longitudinally along the cable. The seismic detectors are transducers that perceive the mechanical activity due to reflected acoustic wavefields and convert that activity to electrical signals having characteristics representative of the intensity, timing and polarity of the acoustic activity as is well known to the art. The detectors are operatively coupled to data-storage and processing devices of any desired type.
An acoustic source such as an array of air guns, is towed in the water by the ship near the leading end of the swath of seismic streamer cables. As the ship proceeds along the line of survey, the source is fired (activated) at selected spatial intervals equal, for example, to a multiple of the seismic detector group spacing, to acoustically illuminate (insonify) the subsurface formations. Assuming the ship travels at a constant velocity such as six knots, the source may be conveniently fired at selected time intervals such as every five seconds, assuming a 50-meter group interval. The wavefield emitted by the source travels downwardly to be reflected from subsea earth formations, whence the wavefield is reflected back to the water surface where the reflected wavefield is received by the detectors and converted to electrical signals as previously explained. The detected electrical signals are transmitted to any well-known signal recording and processing means for providing a physical model of the subsurface.
For a better understanding of a problem to be solved by this disclosure, FIG. 1 shows a source, S, at or near the surface 10 of the water 12. Detectors D.sub.i+1, D.sub.i+2, D.sub.i+3 are disposed near the water surface above a flat-lying formation F. A wavefield emitted from S follows the indicated ray paths to the respective detectors as shown. For example, the ray path from S to D.sub.i+3 is reflected from incident point IP on formation F. The incident angle .phi..sub.i, relative to the perpendicular to F at IP or zero-offset point Z, must equal the angle of reflection .phi..sub.r as in geometric optics, assuming the earth material is isotropic. The surface expression of the subsurface reflection point, R, the midpoint between S and D.sub.i+3, M, and the zero offset point, Z, are coincident. The incident points of all of the raypaths are evenly distributed along the line as shown.
In regions of steep dip, the symmetrical picture of FIG. 1 is distorted as shown in the 2-D illustration of FIG. 2. are, with a dip of 45.degree., while the angles of incidence and reflection .phi..sub.i and .phi..sub.r are equal, the zero-offset point Z, is up-dip of the midpoint M. The surface expression R, of the reflection point (incident point IP) lies not between the source and detector as in FIG. 1, but up-dip of the source S.
FIG. 3 traces a number of raypaths from a source S to detectors D.sub.i-1, D.sub.i+1, D.sub.i+2, D.sub.i+3, D.sub.i+n for a 45.degree.-dipping bed F. The important point to observe in this Figure is the non-uniform spacing of the incident points. Because reciprocity holds, assuming that the earth materials are isotropic, the source and detectors can be interchanged. It is thus evident that when shooting down-dip, the incident points tend to bunch up. Shooting up-dip results in a spreading-apart of the incident points. Because of the complex non-uniform subsurface illumination, significant undesirable shadow zones are formed. The problem becomes particularly troublesome where multiple cables are used in a 3-D swath, due to the additional awkward lateral geometry.
One method for minimizing shadow zones is taught by C. Beasley (co-inventor in the present invention) in U.S. patent application Ser. No. 08/069,565 filed May 28, 1993, entitled, "Quality Assurance for Spatial Sampling for DMO", assigned to the assignee of this invention and issued Sep. 12, 1995 as U.S. Pat. No. 5,450,370 which is incorporated fully herein for all purposes. That application is the basis for a paper delivered in 1993 at the 63rd Annual meeting of the Society of Exploration Geophysicists and published in Expanded Abstracts, pp. 544-547. That invention provided a method for examining the geometry of the disposition of a plurality of sources and receivers over an area to be surveyed with a view to optimizing the array to avoid shadow zones in the data and to optimize the resulting seismic image. The method depends upon studying the statistical distribution of dip polarity in dip bins along selected CMP azimuths. The method was implemented by rearranging the geometrical disposition of the sources and receivers. It was not directed to the per se problem of non-uniform subsurface coverage and shadow zones in the presence of steep dips.
Another discussion directed to symmetric sampling is found in a paper entitled, "3-D Symmetric Sampling" by G. Vermeer, and delivered in 1994 in a paper at the 64th Annual Meeting of the Society of Exploration Geophysicists, Expanded Abstracts, pp 906-909. Here, the authors review the various different shooting geometries involved in land and marine surveys including 2-D, 3-D and 5-D configurations. The presence of non-uniform subsurface insonification is recognized and the need for symmetric sampling to prevent aliasing is emphasized.
M. S. Egan et al., in a paper entitled, "Shooting Direction: a 3-D Marine Survey Design Issue", published in The Leading Edge, November, 1991, pp 37-41 insists that it is important to maintain consistent source-to-receiver trajectory azimuths to minimize shadow zones, imaging artifacts and aliasing in regions of steep dips. They are particularly concerned about 3-D marine surveys in areas where the proposed seismic lines are obstructed by shipping, offshore structures and other cultural obstacles.
There is a need for equalizing the density of the subsurface coverage provided by wide, towed swaths of seismic streamer arrays in the presence of steeply-dipping earth formations in the circumstance where the acoustic source is located at an end of the swath.